Up to the present time, when an original such as a photograph or a picture having a continuous tone image is to be reproduced by printing in large quantities, it has been necessary to convert the continuous tone image of the original to an image formed by dots (called screened halftone or halftone dots) having various sizes (size to area ratio) according to the image density of the original. This screened halftone image is, then, formed on a printing plate in order to reproduce the image of the photograph, picture, etc. in large quantities, using conventional printing plates and inks.
When converting a continuous tone image into a screened halftone image consisting of various size halftone dots, contact screens having a regular density gradient are normally employed. In practice, when a screened halftone image is to be formed, a contact screen is placed directly on a recording material (such as a lith type film) for a screened halftone image, and the original, such as a transparency, having a continuous tone image is exposed by a process camera through the contact screen onto the recording material; a screened halftone image consisting of various size halftone dots is thereby formed on the recording material, the sizes of the dots being a function of the quantity of light corresponding to the density of the original and the density gradient of the contact screen.
In recent years, other methods using a scanning instrument called a scanner and directly converting a continuous tone original to a screened halftone image, without the use of a process camera, have become the main stream method of recording screened halftone images. The scanner used in such a process is called a direct scanner. The methods for recording screened halftone images by use of such direct scanners can be divided into two types; one type is to use a contact screen similar to those used in the aforementioned process of forming a screened halftone image using a process camera so as to generate screened halftone images, and the other type is to electronically generate halftone images by means of an electronic halftone dot generator stored in a direct scanner, thereby eliminating the contact screen.
In the first type of direct scanner mentioned above using a contact screen, an original with continuous tone image is scanned to obtain electronic video signals corresponding to the density of the image. A light beam of a light source which is modulated by the video signals thus obtained is used to scan and expose, through a contact screen, the recording material that is placed in direct contact with the screen, so as to record the screened halftone image. Since this method is similar to the conventional one that uses a process camera, it is a familiar method, but it poses a variety of problems due to its use of a contact screen. As contact screens themselves suffer scratches, stains, etc. in use, they cannot be used repeatedly, and since they are expensive, the cost of this process is high. To obtain a screened halftone image having good quality, it is essential to keep the contact screen and the recording material in sufficiently close contact, but insufficient contact often occurs and results in irregularities in the configuration and the size of the halftone dots. Dirt or dust may be present between the contact screen and the recording material and prevent close contact. Further, the use of a contact screen itself will often produce fringes around halftone dots, similar to the case of optically recorded screened halftone images using a conventional process camera. Accordingly, the size of the halftone dots will become unstable, and, in turn, after the completion of the screened halftone image recording, it will be often required to correct the size of the halftone dots to the desired one by some method such as dot etching.
In addition, since the exposure is made through a contact screen on a recording material, the light source for the exposure is required to have a high power, requiring a higher cost, and the exposure speed (scanning speed) cannot be increased much. It also takes time to set up the contact screen. Thus, a variety of disadvantages are present in this method.
In order to eliminate the aforementioned various disadvantages caused by the use of a contact screen, the second method, which requires no contact screen, has been used.
This latter method electronically generates halftone dots, and a halftone dot-generator is included in direct scanners used for this purpose. Such a generator of the scanner is also called a dot-generator or halftone generator. The use of an electronic halftone dot-generator will solve all of the problems arising from the use of a contact screen, and hard halftone dots without any fringes will be obtained, and the scanning speed will be enhanced; this method is thus advantageous in terms of operability, quality, stability, material costs, etc., and has become the primary method for generating halftone images. A variety of methods for generating halftone dots by means of electronic halftone dot-generators have been devised and put to practical use up to the present time.
In one such method, values of all halftone dots of various sizes (e.g. halftone dots of from 5% to 95% of maximum at 5% interval) are stored in a memory. Values of halftone dots corresponding to the density-related video signal levels obtained by scanning the original, are read out in sequence from the memory, to compare with and to control the exposure light beam from the recording light source and to record the halftone image. In this method wherein the tone reproduction of the halftone image is determined by the number of the values stored in the memory, it will be necessary to increase the number of the values of the halftone dots stored in the memory in order to obtain a smoother tone reproduction, but this, in turn, poses a problem of requiring an increased memory capacity. This method is disclosed in Behane et al. U.S. Pat. No. 3,604,846 and French Pat. No. 1,585,163.
Another method is to divide the unit area of a contact screen used in the conventional methods into minute cells, and allot a specific threshold value of density to the address of each minute cell, store said value in a memory, scan the original and generate a density-related video signal, electronically compare and control whether to expose or not the minute cell of the reproduction according to the density-related video signal, which is obtained by scanning the original, corresponding to the scanned minute cell, and thus record a screened halftone image in sequence. This method is disclosed in Schreiber U.S. patent application Ser. No. 576,851 (Japanese Patent Provisional Pub. No. SHO. 51-138445).
Accordingly, in this method, each halftone dot consists of a group of plural minute cells, the exposure of which is individually controlled by the electronic halftone dot generating signals. The size of each individual minute cell is always constant, and the number of minute cells will vary according to the optic density (highlight middle shadow) of the continuous tone image of the original.
The inventors of the present invention have concentrated their energies on the study of electronic screened halftone image recording and developed a novel apparatus and method for computing out threshold values of density from some functions without storing them in a memory, thereby avoiding the need for a large memory.